I haven't tried all of them, but the ones I did check were ones that had not had posts on them at their source instance for quite a while. A few random examples:
I had 43 failures and 111 successes, so visual inspection wouldn't really help. I kept copies of the error log and the script output in a text file to figure it out later.
I assume that this means these communities haven't had activity since fedia.io opened, and so fedia.io doesn't know they exist? I've always wondered how the first person to subscribe to a community on an instance is able to do that.
And yeah, I'm using "community" to refer to "magazine".